Leetcode-Intersection of Two Linked Lists

Write a program to find the node at which the intersection of two singly linked lists begins.

 

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

 

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA==null || headB==null) return null;

        ListNode s1 = headA;
        ListNode s2 = headB;
        int len1 = 1;
        while (s1.next!=null){
            len1++;
            s1 = s1.next;
        }
        int len2 = 1;
        while (s2.next!=null){
            len2++;
            s2=s2.next;
        }
        s1 = headA;
        s2 = headB;
        if (len1>len2){
            while (len1>len2){
                s1 = s1.next;
                len1--;
            }
        } else if (len1<len2){
            while (len1<len2){
                s2 = s2.next;
                len2--;
            }
        }

        while (s1!=null && s2!=null){
            if (s1==s2)
                return s1;
            s1 = s1.next;
            s2 = s2.next;
        }

        return null;
    }
}